1. Field of the Invention
The invention concerns amplifiers in general and in particular a wideband low noise amplifier for use in low signal strength applications.
2. Related Art
Low noise wide bandwidth amplifiers have numerous applications. One such application is the detection of low level signals from a photodiode activated by an external light source, for example, in a bar code scanner. A conventional low noise wideband photodiode amplifier, as shown in FIG. 1, employs a single port feedback system using off the shelf amplifier technology, for example, a commercially available operational amplifier known as TLC 272. In the example shown in FIG. 1, the amplifier 101 is biased on its non-inverting terminal and its inverting terminal receives a signal from the photodiode and feedback through a resistor R.sub.1 and capacitor C1 from the output terminal.
The desirable results of increasing bandwidth and reducing sensitivity to noise are typically at odds in such designs. For example, random fluctuations produced by thermal agitation of electrons, or Johnson noise, is proportional to the square root of the feedback resistance R.sub.1. Since in such an amplifier the output signal is proportional to the feedback resistance, R.sub.1, the signal to noise ratio is proportional to R.sub.1 /.sqroot.R.sub.1. This signal to noise ratio increases with increasing values of R.sub.1. Unfortunately, as R.sub.1 is increased to improve the signal to noise ratio, the open loop pole of the amplifier formed by feedback resistor R.sub.1 and diode capacitance Cd, combined with the dominant pole of the TLC 272 amplifier work to quickly reduce the phase margin of the circuit, resulting in an unstable amplifier. One solution to this problem is to increase the value of the capacitor C.sub.1 across resistor R.sub.1. Unfortunately, this limits the bandwidth of the amplifier because of a closed loop pole formed by the product of C.sub.1 and R.sub.1. FIGS. 2, 3a and 3b illustrate the maximum bandwidth which can be achieved when R.sub.1 is set to 1 megohm, C.sub.1 is set to 22 picofarads, the diode capacitance C.sub.d equals 75 picofarads and the diode resistance R.sub.d equals 50 megohms. Thus, the performance limitations of conventional amplifiers restrict their utility to either wide bandwidth applications with a high noise tolerance, or low noise applications with a narrow bandwidth.
U.S. Pat. No. 4,535,233 to Abraham discloses a bootstrap transimpedance preamplifier for a fiber optic receiver. FIG. 2c of Abraham illustrates a three OP-AMP circuit with a resistor providing feedback from the output of the third OP-AMP to the non-inverting input of the first OP-AMP. Column 4, lines 47-55 of Abraham discuss limitations that do not exist in an amplifier according to the invention, as disclosed herein. Abraham discloses that because the feedback resistor is larger than the input resistor in other configurations disclosed therein, noise is reduced and sensitivity is increased. Abraham also discloses that the feedback resistor increases the bandwidth limiting effect of the capacitance of the detector. Abraham seeks to address this problem with a bootstrap circuit. Thus, Abraham fails to recognize that by shifting the dominant pole of the overall amplifier to a higher frequency, in the open loop response, the bandwidth limiting effects can be reduced. In Abraham, the first amplifier acts as a current buffer and, because of the absence of negative feedback, there is no attempt to extend the dominant pole to a higher frequency. As discussed further herein, an amplifier according to the invention extends the dominant pole to a higher frequency, thereby eliminating the need for capacitive compensation across the feedback resistor, which may be required in Abraham to maintain stability in view of the disclosure in column 4, lines 47-55 of Abraham.